In measurement and control systems, various types of displacement transducers are used for producing an electrical signal corresponding to angular displacement. The prior art includes a wide variety of such transducers including those which are characterized as resistive, capacitive and inductive. The inductive type of displacement transducer, in general, utilizes a variation of magnetic reluctance to produce a variation of inductance. The variable inductance may be utilized to produce variation of impedance or of transformer coupling.
A particular application for angular displacement transducers is that of torque sensing in power steering systems for automotive vehicles. In this particular application, the variable reluctance transducer is especially well suited to the rugged environment of the automobile steering system.
The prior art includes a variety of variable reluctance displacement transducers. The Demuth U.S. Pat. No. 3,329,012 discloses a torque measuring or control device comprising a pair of metal disks which are mounted on separate shafts, the shafts being connected by a flexure element which permits limited relative rotation. The disks extend into the air gap of a differential transformer and the disks are provided with openings which overlap to an extent depending upon the torque. The degree of overlap determines the coupling of the transformer windings and hence the output voltage.
The Scoppe U.S. Pat. No. 3,340,729 describes an electromagnetic torque meter which utilizes a magnetostrictive sleeve on a rotating shaft with stationary primary and secondary windings adjacent the sleeve. The signal induced in the secondary winding is a function of the permeability of the sleeve which is varied by the torque applied to the shaft.
The Tavis U.S. Pat. No. 3,562,687 describes a variable reluctance transducer especially adapted for pressure sensing. A magnetic core includes spaced legs which extend radially from a center pole to define a U-shaped channel in which an energizing coil is fitted. Enlarged arcuate tabs extend laterally from the ends of the legs to form a segmented outer pole on the core. A movable armature in the form of a diaphragm is spaced from the poles and is displaced toward or away from the poles in accordance with the variations in the fluid pressure being sensed. Accordingly, the reluctance of the magnetic circuit is varied according to pressure variations.
There is a need for an angular displacement transducer of the electrical type which is capable of producing a highly stable displacement signal under all operating conditions. In particular, it must have a reference or zero position which produces a reference signal value which is substantially invariant with ambient conditions such as temperature, shock and vibration so that false signals are not generated. This is especially important for a transducer to be used in automotive power steering to avoid unwanted steering activation. Further, a transducer for power steering should be compact, lightweight and of rugged construction. It should also lend itself to low cost manufacture in mass production.
A general object of this invention is to provide a transducer which meets the foregoing requirements and which overcomes certain disadvantages of the prior art.